Seat belt arrangements for automotive applications are governed by laws and regulations regarding their design and construction. The seat belt arrangement must be safe to ensure proper protection of the passenger and the requirements to achieve this differ from country to country. Some countries require simultaneous latching of the tongues in the buckle to avoid that one tongue is improperly latched. In other countries it is required that the buckle is equipped with ejector means for ejecting the tongues out of the buckle when the release button is engaged. This makes it easier to determine if the tongues are properly latched in the buckle. Buckles in seat belt arrangements for small children (<9 kg) can be made from polymeric material whereas buckles for larger children often comprise metallic load-bearing parts. Various buckles have been designed, that aim to fulfill the national requirements, and some of them will be described below.
A buckle mechanism of the kind described above is for example shown in U.S. Pat. No. 5,267,377. The buckle mechanism according to this patent uses a single latch member for latching of the tongues, where the shoulder seat belts are attached. The single latch member pivots when only one tongue is inserted into the designated cavity. This means that when the second tongue is inserted, the first tongue becomes partially unlatched and might remain in a partial-latched position. This means that the tongue appears to be latched but the only force holding the tongue is the friction from being urged against the internal cavity by the latch spring. This can be very dangerous in a potential collision.
Furthermore, the aforementioned governing laws and regulations for buckles include limits for the required opening force of the buckle. There are both upper and lower limits between which exists only a narrow range of acceptable opening force values. Buckles of the constructions shown in U.S. Pat. No. 5,267,377, U.S. Pat. No. 4,617,705, and U.S. Pat. No. 5,813,097 are subject to further restriction of the range of opening forces due to the accumulation of manufacturing tolerances inherent in their making. This can be seen in the fact that the position and orientation of the latch member is controlled by the position of the bent features in the frame member or other supporting members in each of these devices. Each added positioning feature or member makes the exact position and orientation of the latch less precise. This results in greater difficulty during series production in which all buckles must be manufactured to meet the force range limitations with statistical certainty. These buckles are also sensitive to bending of the frame or other supporting members during dynamic loading as occurs during collisions. In these cases, it may be possible for the latch member to rotate to a less than ideal position.
Another buckle mechanism of the type described above is disclosed in U.S. Pat. No. 5,406,681. This buckle is constructed of polymeric material and used exclusively in seat belt arrangements for small children (<9 kg). While the construction of this buckle avoids the problems associated with the positioning of the latch member, i.e. accumulated tolerances and sensitivity to dynamic bending discussed above, its construction mandates that it only be used in applications for small children, due to the fact that the polymeric latch member is secured to the anchor pin which is then secured to the polymeric outer housing. The crash loads must then be transferred through the housing to the webbing, meaning that the magnitude of the loads it can carry is expressly limited through reliance on the polymeric housing to transfer load to the webbing.